The present invention relates to a crystal oscillator and, more particularly, to a crystal oscillator for generating a reference clock signal which clocks a timepiece included in a battery-powered portable apparatus.
A battery-powered portable telephone or similar portable apparatus is often provided with a timepiece function for user's convenience. Circuits for the timepiece function are mounted on a one-chip integrated circuit (IC), for a miniature and inexpensive configuration. Preferably, a portable apparatus should be so constructed as to turn off its power source when the expected function thereof is not effective, thereby promoting battery saving. The timepiece, however, does not function correctly unless it is powered at all times. To insure accurate operations, the timepiece is usually clocked by a reference clock signal which is generated by a crystal oscillator. It follows that reducing the current to be consumed by the crystal oscillator is decisive in reducing the current consumption of the entire portable apparatus. More specifically, reducing the current consumption of the crystal oscillator is successful in enhancing battery saving. In light of this, it is a common practice to drive the crystal oscillator by a comparatively low voltage when the portable apparatus is not serving the expected function.
However, the problem is that the oscillation frequency of the crystal oscillator changes with the drive, or source, voltage. Should the source voltage be lowered for the battery saving purpose, the oscillation frequency of the crystal oscillator would be changed to prevent the timepiece from achieving sufficient accuracy.